The present invention relates to an electromagnetic flowmeter having a pressure resistant measurement tube made of a heat-resistant material and surrounded by a stainless steel casing. A magnetic device comprising at least pole pieces and windings is disposed in a sealed spaced between these two parts and one or more measuring electrodes are arranged in the measurement tube. Electromagnetic flowmeters of this type are known from German patent DE 34 23 921 A1.
Flowmeters of the aforementioned type are used to measure the rate of fluid flow and are utilized in a variety of fields including, for example, the food industry.
It is essential that the measurement tube is compression resistant, non-corrosive, structurally rigid and also a good electrical insulator. In particular ceramic materials, e.g. Al.sub.2 O.sub.3, exhibit these characteristics, though other materials, such as heat resistant plastic, are also suitable. Generally, the casing is made of stainless steel, usually stainless austentite steel (1.4301) which has highly non-corrosive properties and is readily deformable by deep-drawing. The latter property is important for the manufacture of flowmeters with steel casings because it results in low material costs. For large production runs casings of this type using austentite steel can also be cast.
A drawback of the stainless steel, especially austentite steel, is that it has a significantly higher coefficient of thermal expansion than materials such as ceramic or plastic, of which the measurement tube is made. Moreover it is very difficult to attach a stainless steel casing to a ceramic or plastic tube.
Especially when subjected to a variety of uses, when fluids must be measured over a wide range of temperatures, it is important to provide a cost effective and reliable seal between the measurement tube and the stainless steel casing. Moreover, the seal must be non-corrosive and the casing must be of the type that can be attached to the measurement tube in a cost-effective manner.
The seal of the flowmeter disclosed in German patent publication DE 34 239 21 A1 employs an expensive O-ring and the attachment occurs when the space surrounding the measurement tube is sealed. An O-ring seal has the further disadvantage that its non-corrosive properties are determined by the material of the O-ring.